Edmar P. Marques

Surface electrochemistry of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide ([MTT]Br) adsorbed on a graphite electrode

Abstract 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide ([MTT]Br), which is of importance as a biological redox indicator, irreversibly adsorbs on a graphite electrode and exhibits two surface electrochemical redox processes. The mechanism of reduction and re-oxidation of this species on the surface is discussed as a function of pH. AM1 calculations and redox data are interpreted to show that protonation of the first reduced product leads to a ring-opening pathway, eventually generating the two-electron reduced formazan species. Cyclic voltammetry, rotating-disk voltammetry, pH dependence studies and electronic spectroscopy are used to characterize the system.

Graphite electrodes modified by 8-hydroxyquinolines and its application for the determination of copper in trace levels

Surface modification by 8-hydroxyquinoline-5-sulfonic acid (8-HQS) or 8-hydroxyquinoline (8-HQ) on a graphite electrode through irreversible adsorption is reported in this paper. Cyclic voltammetry was used to characterize the surface behavior. The modified surface exhibited an affinity to chelating Cu(II) in the solution, forming a Cu(II) complex, which was employed for Cu(II) trace analysis. Of the metals Zn, Ni, Pb, Co, and Cd, none presented interference until excess concentration of 10 times. Significant interference could be observed from Co(II), Cd(II) and Fe(II) for an excess concentration of 100 times on the analyte. A differential pulse voltammetry, combined with a preconcentrating-stripping process and a standard addition method was used for the analysis. A detection limit for trace copper determination in water, such as 5.1´10-9 mol L-1, was obtained.

Electrochemical behavior of ruthenium-hexacyanoferrate modified glassy carbon electrode and catalytic activity towards ethanol electrooxidation

3- ion and three ruthenium forms (Ru(II), Ru(III) and Ru(IV)), characteristic of ruthenium oxide compounds. The modified electrode displayed excellent electrocatalytic activity towards ethanol oxidation in the potential region where electrochemical processes Ru(III)-O-Ru(IV) and Ru(IV)-O-Ru(VI) occur. Impedance spectroscopy data indicated that the charge transfer resistance decreased with the increase of the applied potential and ethanol concentration, indicating the use of the RuHCF modified electrode as an ethanol sensor. Under optimized conditions, the sensor responded linearly and rapidly to ethanol concentration between 0.03 and 0.4 mol L -1 with a limit of detection of 0.76 mmol L -1 , suggesting an adequate sensitivity in ethanol analyses.

Application of electrochemical impedance spectroscopy: A phase behavior study of babassu biodiesel-based microemulsions.

Microemulsions are thermodynamically stable systems of two immiscible liquids, one aqueous and the other of organic nature, with a surfactant and/or co-surfactant adsorbed in the interface between the two phases. Biodiesel-based microemulsions, consisting of alkyl esters of fatty acids, open a new means of analysis for the application of electroanalytical techniques, and is advantageous as it eliminates the required pre-treatment of a sample. In this work, the phase behaviours of biodiesel-based microemulsions were investigated through the electrochemical impedance spectroscopy (EIS) technique. We observed thatan increase in the amount of biodiesel in the microemulsion formulation increases the resistance to charge transfer at the interface. Also, the electrical conductivity measurements revealed that a decrease or increase in electrical properties depends on the amount of biodiesel. EIS studies of the biodiesel-based microemulsion samples showed the presence of two capacitive arcs: one high-frequency and the other low-frequency. Thus, the formulation of microemulsions plays an important role in estimating the electrical properties through the electrochemical impedance spectroscopy technique.

Novo método espectrofotométrico para determinação de Hg (II) em amostras de peixe

A simple, sensitive and selective spectrophotmetric method is presented for the rapid determination of mercury (II) at trace levels using 2-mercapto-5-methylbenzimidazole (MMBI) as a new spectrophotometric reagent (lambdamax = 320 nm) in a slightly basic (0.04 M Britton-Robinson (BR) buffer, pH 8.0) aqueous solution. The reaction is instantaneous and the absorbance remains stable for over 24 hours. The average molar absorption coefficient was found to be 2.71 x 104 L mol-1 cm-1. The stoichiometric composition of the chelate is 1:1 (Hg:MMBI). Considerable excesses of copper, zinc, lead and cadmium do not interfere in the determination. The method developed allowed the determination of mercury in the range of 2 x 10-6 to 4 x 10-5 mol.L-1 with good precision, accuracy and the detection limit was 9.9 x 10-7 mol.L-1 of Hg. The method was successfully applied to fish samples and the results were evaluated and compared with classic atomic absorption spectrometry (AAS). The relative standard deviations for the samples analyzed were between 7.2 and 33% (n = 5), while the errors were between 1.63 and 11.6. The method was found to be sensitive, selective and was used for the determination of mercury in fish with satisfactory results.

Distribution and Fractionation of Metals in Mangrove Sediment from the Tibiri River Estuary on Maranhão Island

Sediment quality guidelines and the sequential chemical extraction protocol of the European Commission were applied for the assessment of Cd, Cr, Cu, Ni, Pb and Zn in mangrove sediments from the Tibiri River on Maranhao Island. Metals concentrations were determined using the inductively coupled plasma optical emission spectrometry (ICP OES) technique. The results demonstrated that metal levels were below TEL (Threshold effect level) and ERL (effect range low), range associated with occasional adverse biological effects for the aquatic life. Metal mobility in decreasing order was: Ni (24%) > Zn (20.6%) > Cu (14.5%) > Pb (1.9%). Except by Ni and Zn, other metals were not found in the exchangeable/acid-soluble fraction (F1). Only Ni, Zn and Cu were present in the reducible fraction (F2). The major metal percentage was associated to oxidizable fraction (F3) and residual fraction (F4). In the latter, metal percentage ranged from 76 to 100%, which suggests an inexpressive impact of anthropogenic activities.

Avaliação integrada da qualidade de águas superficiais: grau de trofia e proteção da vida aquática nos rios Anil e Bacanga, São Luís (MA)

This work proposes two indexes to assess the quality of surface waters where domestic sewage or similar pollution source prevails: the Trophic State Index (IETP) and the index for protection of aquatic life (IQApva). Both were applied to Anil and Bacanga rivers located on Maranhao Island. Although the general quality of the water indicates human presence, the Anil River stands out by its high trophic level and poor conditions for aquatic life. The results of applying these indexes allow easily assess of the degradation state of the ecosystem. By being connected directly to basic sanitation, these indices are an important management tool for governmental and non-governmental stakeholders.

New Pb2+ carbon paste electrode based on organically modified silicate and its square wave anodic stripping voltammetric response for pretreated gasoline samples

An analytical procedure for determining Pb2+ by square wave anodic stripping voltammetry in pretreated gasoline sample is described and a carbon paste electrode modified with a inorganic-organic hybrid material (aminopropyl linked to the silica network (APSil) obtained by the sol-gel method) was evaluated as an electrochemical sensor. The optimum conditions for the analysis of Pb2+ ions were presented as follows: composition of APSil on the carbon paste electrode 40% (wt.%); deposition solution of pH 1.8; accumulation potential of -0.7 V vs. Ag/AgCl and deposition time of 600 s. Interference studies indicated that only Cu2+, at a 5-fold excess, interfered drastically on the stripping response of Pb2+.

Identificação e quantificação do cristal violeta em aguardentes de mandioca (tiquira)

Tiquira is a traditional homemade alcoholic distillate produced in the Maranhao State (Brazil), gotten from cassava (Manihot esculenta, Crantz.). It can be normally found on street markets. Due to the addition of tangerine leaves, the original tiquira has a bluish color. Samples of this beverage were acquired in the local trade and analyzed from the spectroanalytical point of view. The results indicated that these drinks had been adulterated by the addition of crystal violet, a potencialy hazardous compound. The identification and quantification of crystal violet in 10 spiked samples was accomplished by UV-Vis spectrophotometry through the standard addition method. In order to verify the efficiency of the proposed method, experiments on the quantification and recovery were carried out and the results indicated a content of crystal violet in the 10-6 to 10-7 mol L-1 range.

A simple and fast method to determine water content in biodiesel by electrochemical impedance spectroscopy

In this paper, a fast, simple and new alternative method for determination of water content (moisture) in biodiesel using electrochemical impedance spectroscopy (EIS) is developed. The method is based on the sensitive variation of the charge transfer resistance (Rct) of the biodiesel medium in the presence of different quantities of water. To obtain an accurate analytical measurement, a pre-treatment based on a simple dilution by acetonitrile is employed. The two identical Pt electrodes are used in the measurement cell filled with sample solution. The experiment conditions are also optimized for the measurements. The obtained analytical linear curve between the water content and EIS impedance is the basis for the water content analysis in biodesiel fuel. The EIS method is then successfully applied to both real and certified samples, and the results confirm that the method is reliable with high sensitivity, precision and accuracy. The comparison of EIS method with the official standard method is also made through the Student test t, demonstrating that both methods are statistically consistent and similars. The validation of such an EIS method confirms that the method presented for the fist time in this paper can be succesessfully applied to determining the water content of biodiesel fuel.